1. Technical Field
The present disclosure is related to a power converter, and in particular, to a flyback active clamping power converter making a diode in a secondary side perform Zero Current Switching (ZCS), preferably a single-ended primary inductance converter.
2. Description of Related Art
With the advancement of technology and the rapid development of portable electronic products, more and more attention has been paid to the performance of switching converters and each of their applications. In recent years, due to major advances in the power electronics technology and the development of nanotechnology, a growing tendency of electronic devices has been towards being slim and light, energy saving, and cost-reducing, so its internal power converter design also needs to trend to be slim and light, energy saving, power increasing, and cost-reducing. A power supply is generally required for most of electric appliances, to convert an input power source from batteries or AC power grids into an output power source with specific ratings. As technology advances, it has become routine for power supplies to operate more efficiently or have higher conversion efficiency. As known in the art, the conversion efficiency of a power supply is defined as the ratio of the output power from the output power source to the input power from the input power source.
A power conversion system receives an input DC or AC power and converts the input DC or AC power to a DC or AC output power, and the DC or AC output power usually presents a voltage different from the input power. Control of the output power may respond to the output voltage or the output current. A boost converter (also known as step-up converter) is a power converter in which the output voltage is greater than the input voltage. The boost converter includes at least a first electrically controlled switch (for example, a transistor), at least a first energy-storing element (for example, an electric coil) and a switched mode power for an add-on (for example, a diode or a second electrically controlled switch). Generally, the electrically controlled switches and the diodes are configured between the coil and the output end, wherein the current alternately charges the coil, in response to a turning-off operation of the first electrically controlled switch, and the current is transmitted to a load, in response to a turning-on operation of the first electrically controlled switch. When the current is transmitted to the load, the current is flowing through the diode or the second electrically controlled switch. A buck converter (also known as a step-down converter) is a power converter in which the input voltage is greater than the output voltage. The buck converter includes at least a third electrically controlled switch (for example, a transistor), at least a second energy-storing element (for example, an electric coil) and a switched mode power source for an add-on (for example, a diode or a fourth electrically controlled switch). Generally, the electrically controlled switches and the diodes are configured between the input power source and the coil, wherein the current alternately draws and charges the electric coil via a load, in response to a turning-off operation of the third electrically controlled switch, and the electric coil continuously releases energy to the load, in response to a turning-on operation of the third electrically controlled switch. When the coil releases energy to the load, the diode or the fourth electrically controlled switch is connected to the coil in series. A flyback converter is a power converter in which the output voltage can be greater or less than the input voltage. The flyback converter is a switched mode power which includes at least an electrically controlled switch; an energy-storing element which includes at least a coil (especially a transformer), whereby a voltage ratio is increased with an additional advantage of isolation; and at least an add-on (for example, a diode and/or an additional electrically controlled switch). Generally, a primary coil of the transformer is connected between the electrically controlled switch and the input voltage, and a secondary coil of the transformer is connected between the add-on and the output voltage. As used herein, the above-mentioned power converters are used for explanation of some topologies, but are not limited thereto.
In the low-power applications, the flyback converter is a widely used power circuit, but the flyback converter has the disadvantages of input current pulse sufferring from electromagnetic interference (EMI) and voltage spikes caused by leakage inductance. Therefore, in the prior art, the single-ended primary inductor converter has been used to reduce the disadvantage of electromagnetic interference. However, the traditional active clamping single-ended primary inductor converter (SEPIC) has a higher spike voltage in the diode in the secondary side, thus the efficiency cannot be greatly improved.